1. Field of the Invention
This invention relates to the cancellation of echo signals from transmitted digital data. In particular, it relates to the cancellation of echo signals from digital data transmitted over two-way, two-wire telephone transmission channels.
2. Description of the Prior Art
In the field of data communications, it is often advantageous for traffic to be carried over a single communication line (link, channel) in two directions simultaneously. That is, the traffic is full-duplex. A typical transmission medium is a two-wire telephone channel within the public switched direct-distance-dialing (DDD) network. The passband of such a two-wire channel extends from approximately 300 to 3000 hertz. For full-duplex data transmission the available bandwidth can be divided in half, with each half being allocated to a particular transmission direction. However with this method, accurate data transmission can only be achieved at half the rate that could be achieved in one-way (half-duplex) transmission. One way to increase the full-duplex data rate is to use two physically separate two-wire lines, with each line carrying a full bandwidth one-way signal in a respective one of the two transmission directions. This is referred to as a four-wire channel.
Alternatively, high speed, full-duplex transmission can be carried out over a single two-wire channel by using hybrid coupling networks. These networks, positioned at both the so-called near and far ends of the two-wire channel, accept a four-wire signal and convert it into a two-wire signal for transmission over a two-way, two-wire telephone channel. For optimally interference-free transmission, the impedance of the port of the hybrid which interfaces with the channel must exactly match the impedance of the two-wire channel. In practice, however, this is seldom possible.
Specifically, the switched nature of the DDD network means that a large number of communication channels of differing impedance are connected over time to the hybrid. Because the hybrid is designed to operate over as many differing communication channels as practicable, there is generally a mismatch between the hybrid and the channel. Such a mismatch causes a portion of the signal which was transmitted from the near end to be reflected from the channel/far-end hybrid connection point back into the channel. As in voice transmission, this distant reflected signal is referred to as echo. A data receiver is typically unable to distinguish between data from the far end and the echo of data from the near end. Thus, there is a potential for the near-end receiver to erroneously interpret the echo reflected from the far end as far-end data.
This problem can be handled through the use of echo cancellers. These produce a signal which is essentially a replica of the echo component present in an incoming signal, i.e., the signal applied from the two-wire channel to the near-end hybrid. Specifically, each of a predetermined number of previous consecutive symbols in the transmitted signal, in addition to being transmitted, is stored in the echo canceller. Each such symbol is multiplied therein by a respective tap coefficient. The resulting products are summed to produce the replica signal. A resultant substantially echo-free signal, hereinafter referred to as an echo compensated signal, is obtained by subtracting the replica signal from the incoming signal. The echo compensated signal is applied to a data receiver, which, after processing such as equalization and demodulation, forms decisions as to the values of the transmitted data symbols.
In general, the echo cancellation process is not perfect. Rather the echo compensated signal may contain an uncancelled echo component. It may also contain a far-end data component as described more fully below. In either case, the magnitude of the uncancelled echo component is indicative of the current effectiveness of the echo cancellation process. A large uncancelled echo component means that the replica signal is an inaccurate replication of the echo component sought to be cancelled. In so-called adaptive echo cancellers, the echo compensated signal is advantageously used as an error signal in response to which the values of all of the tap coefficients are adaptively updated in such a way that the uncancelled echo component is minimized. This assures that the replica signal continuosly and, to the extent possible, accurately duplicates the echo component present in the incoming signal, even if the characteristics of the channel change.
The arrangement taught in U.S. Pat. No. 4,087,654 issued May 2, 1978 to K. H. Mueller is illustrative of the so-called baud rate adaptive echo cancellers. In these structures, sampling of the incoming signal, replication of the echo component and echo cancellation all occur at the baud (symbol) rate. Although possessing structural simplicity, these cancellers are highly sensitive to variations in the synchronous timing between the near-end transmitted signal, which is used to define the echo signal replica, and the received data, whose timing is determined at the far end. Moreover, the echo-compensated signal is available to the reveiver only at the baud sampling rate. This severely restricts the receiver's ability to accurately recover timing from the far-end signal.
Alternatively, canceller operation at the Nyquist rate has been suggested. Nyquist sampled schemes, which alleviate the above-described timing problem, are exemplified by S. B. Weinstein in U.S. Pat. No. 4,131,767 issued Dec. 26, 1978 and in "A Passband Data-Driven Echo Canceler for Full-Duplex Transmission on Two-Wire Circuits", IEEE Transactions on Communications, Vol. COM-25, No. 7, July 1977, pages 654-666, and by K. H. Mueller in " A New Digital Echo Canceler for Two-Wire Full-Duplex Transmission", IEEE Transactions on Communications, Vol. COM-24, No. 9, September 1976, pages 956-962. In contrast to baud rate cancellers, Nyquist rate cancellers perform sampling of the incoming signal, echo replica generation and echo cancellation all at the Nyquist rate. Tap coefficient adaptation in the Nyquist arrangements is satisfactory in full-duplex systems during intervals of one-way transmission. However, adaptation is unreliable during intervals of double talk, or two-way transmission, i.e., the simultaneous transmission of far-end and near-end data. These problems arise because the echo compensated signal which feeds the adaptive structure contains not only the uncancelled echo component during double-talk intervals, but also a far-end data component. The far-end data is uncorrelated with respect to the echo. Adaptation and, hence, echo replica generation in response to this signal are thus either unreliable and inaccurate, or is very slow. Thus erroneous data recovery may result. (Baud rate structures are unaffected by these problems because the error signal used to update the echo canceller tap coefficients is taken from a different point in the system, where the far-end data has been determined, and hence has been subtracted out. As such, the error signal is not corrupted by the presence of far-end data.)
Prior art solutions to the above-mentioned problems with Nyquist cancellers include the use of a double-talk detection circuit to halt adaptation and freeze the tap coefficients to their pre-double-talk values for use during the double-talk intervals. See, for example, U.S. Pat. No. 3,499,999 issued Mar. 10, 1970 to M. M. Sondhi. Alternatively, as disclosed by Weinstein in the above-cited paper, a running average of a predetermined number of prior coefficient values for each tap can be used in place of the adaptive coefficients during double-talk intervals. While these solutions stabilize system operation, echo cancellation during the double-talk intervals is potentially inaccurate. This is due to the inability of the tap coefficients to adjust during the double-talk intervals to changes in the echo channel impulse response occurring during those intervals.